In recent electronic devices and the like, optical wirings are employed in addition to electric wirings to cope with increase in information transmission amount. With a trend toward size reduction of the electronic devices and the like, there is a demand for a wiring board which has a smaller size and a higher integration density so as to be mounted in a limited space. An opto-electric hybrid board as shown in FIG. 10, for example, is proposed as such a wiring board. In the opto-electric hybrid board, an opto-electric module portion E including an electric wiring 13 of an electrically conductive pattern and an optical element 10 is provided on each (or one) of opposite end portions of a front surface of an insulation layer 12 such as of a polyimide, and an optical waveguide W (interconnection portion) including an under-cladding layer 20, a core 21 and an over-cladding layer 22 is provided on a back surface of the insulation layer 12 (opposite from the surface of the insulation layer 12 formed with the electric wiring 13) (see, for example PTL 1).
In the opto-electric hybrid board, a light signal transmitted through the core 21 of the optical waveguide W as shown by a one-dot-and-dash line P in FIG. 10 is converted into an electric signal by the optical element 10 of the opto-electric module portion E for electrical control. Further, an electric signal transmitted through the electric wiring 13 is converted into a light signal by the optical element 10. The light signal is transmitted through the optical waveguide W to another opto-electric module portion (not shown) provided on an opposite side, and taken out as an electric signal again.
In the opto-electric hybrid board, the insulation layer (such as of the polyimide) 12 contacts the optical waveguide W (such as made of an epoxy resin). Therefore, the optical waveguide W is liable to be stressed or slightly warped due to a difference in linear expansion coefficient between the insulation layer 12 and the optical waveguide W at an ambient temperature. This increases the light transmission loss of the optical waveguide W.
To cope with this, a metal reinforcement layer 11 such as of stainless steel is provided on the back surface of the insulation layer 12 in the opto-electric module portion E, whereby the stress and the slight warpage of the optical waveguide W are prevented to suppress the increase in light transmission loss. Without the provision of the metal reinforcement layer 11 in a portion of the opto-electric hybrid board other than the opto-electric module portion E, it is possible to ensure the flexibility of the optical waveguide W, so that the opto-electric hybrid board can be mounted in a smaller space to establish optical and electrical connections in a complicated positional relationship.